1. Field of the Invention
The invention relates generally to hydraulically damped mounts and, more particularly, to hydraulically damped elastic mounts for motor vehicles.
2. Description of the Prior Art
Such elastic mountings are used to provide support for engines in motor vehicles of all types. For the support of internal combustion motors in motor vehicles, it is necessary to provide, on the one hand, the softest possible mounting with a low self-damping characteristic to prevent the transmission of noise. These mountings allow the motor movements excited by the roadway to become very large and potentially excessive and then slowly to almost die out. These large motor movements, on the other hand, can be reduced by hard bearings or separate vibration dampers, but that solution results in significant transmission of noise to the body of the vehicle.
Elastic mounts of the general type of the present invention are known, for example, in German Laid Open Patent Application No. DE-OS 32 46 587, which have good damping in the low frequency range. A disadvantage, however, is that above a certain amplitude of vibration or a certain frequency in the decompression phase, cavitation occurs in the chamber, which leads to an undesirable production of noise. Such cavitation always occurs when, with large amplitudes, the damping fluid flows from the one chamber into the other chamber, and a vacuum is formed ahead of the throttle point, since the fluid equalization cannot take place rapidly enough. In the compression stage, on the other hand, undesirable noise is also caused, when large amplitudes occur, by the damping fluid again not flowing quickly enough from the one chamber into the other chamber, because of inertia action of the fluid. The throttle then is essentially blocked, and because of the excessive pressure increase, noises again occur, since the pressure peaks which occur at large amplitudes cannot easily be reduced.
Cavitation of a liquid results when the pressure in a point in the liquid decreases to a value which is below the vapor pressure of the fluid. When the vapor pressure of a fluid is greater than the pressure at a point therein, the fluid changes from the fluid phase, that is, liquid phase, into the vapor phase.
This change of phase is akin to boiling. The vapor will be produced during cavitation as long as the pressure within the fluid is below that of its vapor pressure. Of course, the vapor pressure of fluids within hydraulic damping equipment causes cavitation at pressures well below atmospheric pressure, that is, the fluids boil at operating temperatures.
In the newer and smaller motors, which are generally of four-cylinder design, the vibrations in the motor are at substantially half the frequency of the vibrations of an eight-cylinder motor running at the same number of revolutions per minute. Therefore, the frequency of excitation of the motor is at generally lower frequencies than in corresponding six- and eight-cylinder motors. Furthermore, low frequency vibrations especially during start-up and shut-down of the engine are more prevalent because of the inherent nature of the lower frequencies of vibration due to the smaller number of piston explosions, etc. Additionally, the relative weight of the pistons to the entire engine in a four-cylinder motor are generally greater than in a larger motor. A four-cylinder motor generates greater amplitudes of vibration. Since these greater amplitudes of vibration are at a lower frequency, as is well known in the vibration theory, these lower frequency vibrations will have greater amplitudes than similar higher frequency vibrations of similar energy, because the low pass characteristics of a smaller engine are not as highly filtered as those of a larger engine. Therefore, in smaller engines which are lighter in weight and do not damp out the frequencies of oscillation the same way that larger motors with a greater number of cylinders do, the danger of cavitation in the engine mounts is greater. Because of the problems which have developed in the trend towards lighter, smaller cars with correspondingly lighter, smaller motors with less cylinders, there has been a growing need for refinement of the engine mounts.